A cardiac sodium channel mutation cosegregates with a rare connexin40 genotype in familial atrial standstill

Two polymorphisms in the Cx40 promoter are associated with hypertension and left ventricular hypertrophy preferentially in men

BACKGROUND

Lack of connexin40, a gap junction protein expressed in endothelial and renin-producing cells, results in hypertension and cardiac hypertrophy in mice due to unleashed renin production caused by disruption of the pressure-induced feedback inhibition. We analysed human GJA5 consisting of two exons (exon1A or 1B and exon2) in a selected cohort identified by a single nucleotide polymorphism (SNP) in the GJA5 intron for polymorphisms and putative association with hypertension and left ventricular hypertrophy (LVH).

METHODS

Individuals carrying a SNP in the intron of GJA5 (rs791295) were selected from the MONICA/KORA cohort (n = 1677) and searched for GJA5 polymorphisms. We accessed DNA of 178 probands, of which 26 suffered from LVH, 112 were hypertensive and 29 normotensive (unknown: 11).

RESULTS

Sequencing of the GJA5 coding region did not reveal alterations suggesting the expression of functional connexin40 in all probands. Sequencing of the upstream region of transcript 1A including exon1A revealed two previously described linked SNPs (rs35594137 -44G>A; rs11552588 + 71A>G) at an increased frequency. Moreover, the rare genotype was significantly associated with hypertension and LVH with a preponderance in men. Functional analysis in a reporter gene assay verified promoter activity, however, it was unchanged by the identified SNPs after expressing respective reporter constructs in HeLa and human endothelial cells.

CONCLUSION

We suggest to consider the -44G>A SNP upstream of the connexin40 transcript 1A indeed as a risk factor for hypertension in men. However, the underlying mechanisms remain unclear but animal data suggest that renin-producing cells may be involved and contribute to hypertension.

Gain-of-function mutation in TASK-4 channels and severe cardiac conduction disorder

Analyzing a patient with progressive and severe cardiac conduction disorder combined with idiopathic ventricular fibrillation (IVF), we identified a splice site mutation in the sodium channel gene SCN5A. Due to the severe phenotype, we performed whole-exome sequencing (WES) and identified an additional mutation in the KCNK17 gene encoding the K_2P potassium channel TASK-4. The heterozygous change (c.262G>A) resulted in the p.Gly88Arg mutation in the first extracellular pore loop. Mutant TASK-4 channels generated threefold increased currents, while surface expression was unchanged, indicating enhanced conductivity. When co-expressed with wild-type channels, the gain-of-function by G88R was conferred in a dominant-active manner. We demonstrate that KCNK17 is strongly expressed in human Purkinje cells and that overexpression of G88R leads to a hyperpolarization and strong slowing of the upstroke velocity of spontaneously beating HL-1 cells. Thus, we propose that a gain-of-function by TASK-4 in the conduction system might aggravate slowed conductivity by the loss of sodium channel function. Moreover, WES supports a second hit-hypothesis in severe arrhythmia cases and identified KCNK17 as a novel arrhythmia gene.

Asymmetric functional contributions of acidic and aromatic side chains in sodium channel voltage-sensor domains

Conserved acidic and aromatic residues in the four sodium channel voltage-sensor domains make domain-specific functional contributions.

Voltage-gated sodium (Na_V) channels mediate electrical excitability in animals. Despite strong sequence conservation among the voltage-sensor domains (VSDs) of closely related voltage-gated potassium (K_V) and Na_V channels, the functional contributions of individual side chains in Na_v VSDs remain largely enigmatic. To this end, natural and unnatural side chain substitutions were made in the S2 hydrophobic core (HC), the extracellular negative charge cluster (ENC), and the intracellular negative charge cluster (INC) of the four VSDs of the skeletal muscle sodium channel isoform (Na_V1.4). The results show that the highly conserved aromatic side chain constituting the S2 HC makes distinct functional contributions in each of the four Na_V domains. No obvious cation–pi interaction exists with nearby S4 charges in any domain, and natural and unnatural mutations at these aromatic sites produce functional phenotypes that are different from those observed previously in K_v VSDs. In contrast, and similar to results obtained with K_v channels, individually neutralizing acidic side chains with synthetic derivatives and with natural amino acid substitutions in the INC had little or no effect on the voltage dependence of activation in any of the four domains. Interestingly, countercharge was found to play an important functional role in the ENC of DI and DII, but not DIII and DIV. These results suggest that electrostatic interactions with S4 gating charges are unlikely in the INC and only relevant in the ENC of DI and DII. Collectively, our data highlight domain-specific functional contributions of highly conserved side chains in Na_V VSDs.

Stroke in a Young Individual with Left Ventricular Noncompaction and Left Atrium Standstill

Isolated left ventricular noncompaction (LVNC) is a rare cardiomyopathy with morphologic characteristics of two distinct myocardial layers i.e., thin compacted epicardial and thick noncompacted endocardial layers. The noncompacted myocardium consists of prominent ventricular trabeculae and deep intertrabecular recesses. It can lead to arrhythmias, heart failure or systemic embolisms. Electrocardiographic patterns of patients with LVNC are various and non-specific; however, the most common findings are intraventricular conduction delay, left ventricular hypertrophy, and repolarization abnormalities. We reported the first case, to the best of our knowledge, of a 29-year-old man who had recent cerebral infarction and incidental LVNC with spontaneous left atrial standstill.

Compound heterozygous mutations in the SCN5A-encoded Nav1.5 cardiac sodium channel resulting in atrial standstill and His-Purkinje system disease

An 11-year-old girl on evaluation for syncope was found to have progressive sinus node dysfunction and His-Purkinje system disease with atrial standstill. Genetic analysis revealed compound heterozygous mutations of the SCN5A gene in a novel combination.

Cardiac sodium channel mutations: why so many phenotypes?

Mutations of the cardiac sodium channel (Nav1.5) can induce gain or loss of channel function. Gain-of-function mutations can cause long QT syndrome type 3 and possibly atrial fibrillation, whereas loss-of-function mutations are associated with a variety of phenotypes, such as Brugada syndrome, cardiac conduction disease, sick sinus syndrome, and possibly dilated cardiomyopathy. The phenotypes produced by Nav1.5 mutations vary according to the direct effect of the mutation on channel biophysics, but also with age, sex, body temperature, and between regions of the heart. This phenotypic variability makes genotype-phenotype correlations difficult. In this Perspectives article, we propose that phenotypic variability not ascribed to mutation-dependent changes in channel function might be the result of additional modifiers of channel behaviour, such as other genetic variation and alterations in transcription, RNA processing, translation, post-translational modifications, and protein degradation. Consideration of these modifiers might help to improve genotype-phenotype correlations and lead to new therapeutic strategies.

Mutations in cardiovascular connexin genes

Connexins (Cxs) form a family of transmembrane proteins comprising 21 members in humans. Cxs differ in their expression patterns, biophysical properties and ability to combine into homomeric or heteromeric gap junction channels between neighbouring cells. The permeation of ions and small metabolites through gap junction channels or hemichannels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. Among others, Cx37, Cx40, Cx43, Cx45 and Cx47 are found in heart, blood and lymphatic vessels. Mutations or polymorphisms in the genes coding for these Cxs have not only been implicated in cardiovascular pathologies but also in a variety of other disorders. While mutations in Cx43 are mostly linked to oculodentodigital dysplasia, Cx47 mutations are associated with Pelizaeus-Merzbacher-like disease and lymphoedema. Cx40 mutations are principally linked to atrial fibrillation. Mutations in Cx37 have not yet been described, but polymorphisms in the Cx37 gene have been implicated in the development of arterial disease. This review addresses current knowledge on gene mutations in cardiovascular Cxs systematically and links them to alterations in channel properties and disease.

Whole-exome sequencing identifies Y1495X of SCN5A to be associated with familial conduction disease and sudden death

SCN5A mutations have been reported to underlie a variety of inherited arrhythmias, while the complex overlapping phenotype, especially with congenital heart disease (CHD), is rarely reported. The 48-year-old proband underwent a recent syncope during rest. A CHD (tetralogy of Fallot) and conduction disease was revealed by echocardiogram and ultrasonic cardiogram examination. We combined whole-exome sequencing (WES) and bioinformatics strategies to identify the pathogenic gene for this autosomal-dominant cardiac conduction disease (CCD) in a multi-generation pedigree. We examined four members of this family, including three affected and one unaffected. A novel nonsense mutation (Y1495X) in SCN5A was identified in the affected family members. This mutation is predicted to generate a truncated SCN5A protein, which could result in the loss of sodium current, a defined mechanism of SCN5A related arrhythmias. Our study provides evidence that WES is a highly effective approach for genetic analyses of rare clinical phenotypes. Our study also offers accurate genetic testing information for those yet clinically negative relatives.

Cardiac sodium channels and inherited electrophysiological disorders: an update on the pharmacotherapy

INTRODUCTION

Since the recognition of inherited sodium (Na(+)) channel disease, the cardiac Na(+) channel has been extensively studied. Both loss-of-function and gain-of-function mutations of the cardiac Na(+) channel are associated with cardiac arrhythmia and sudden cardiac death. Pathophysiological mechanisms that may induce arrhythmia are unravelled and include alterations in biophysical properties due to the mutation in SCN5A, drug use and circumstantial factors. Insights into the mechanisms of inherited Na(+) channel disease may result in tailored therapy. However, due to the complexity of cardiac electrical activity and pathophysiological mechanisms, pharmacotherapy in cardiac Na(+) channel disease remains challenging.

AREAS COVERED

This review discusses various mechanisms involved in inherited Na(+) channel disorders, focussing on Brugada syndrome (Brs) and long QT syndrome type 3 (LQTS3). It aims to provide an overview of developments in pharmacotherapy, discussing both treatment and which drugs to avoid to prevent arrhythmia.

EXPERT OPINION

Altered biophysical properties of cardiac Na(+) channels are the basis of arrhythmias in patients with inherited Na(+) channel diseases such as BrS and LQTS3. The effects of such biophysical derangements are strongly modulated by concomitant factors. Tailored drug therapy is required to prevent arrhythmia and is best achieved by educating patients affected by Na(+) channel disorders.

Sequencing of SCN5A identifies rare and common variants associated with cardiac conduction: Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Consortium

BACKGROUND

The cardiac sodium channel SCN5A regulates atrioventricular and ventricular conduction. Genetic variants in this gene are associated with PR and QRS intervals. We sought to characterize further the contribution of rare and common coding variation in SCN5A to cardiac conduction.

METHODS AND RESULTS

In Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) Consortium Targeted Sequencing Study, we performed targeted exonic sequencing of SCN5A (n=3699, European ancestry individuals) and identified 4 common (minor allele frequency >1%) and 157 rare variants. Common and rare SCN5A coding variants were examined for association with PR and QRS intervals through meta-analysis of European ancestry participants from CHARGE, National Heart, Lung, and Blood Institute's Exome Sequencing Project (n=607), and the UK10K (n=1275) and by examining Exome Sequencing Project African ancestry participants (n=972). Rare coding SCN5A variants in aggregate were associated with PR interval in European and African ancestry participants (P=1.3×10(-3)). Three common variants were associated with PR and QRS interval duration among European ancestry participants and one among African ancestry participants. These included 2 well-known missense variants: rs1805124 (H558R) was associated with PR and QRS shortening in European ancestry participants (P=6.25×10(-4) and P=5.2×10(-3), respectively) and rs7626962 (S1102Y) was associated with PR shortening in those of African ancestry (P=2.82×10(-3)). Among European ancestry participants, 2 novel synonymous variants, rs1805126 and rs6599230, were associated with cardiac conduction. Our top signal, rs1805126 was associated with PR and QRS lengthening (P=3.35×10(-7) and P=2.69×10(-4), respectively) and rs6599230 was associated with PR shortening (P=2.67×10(-5)).

CONCLUSIONS

By sequencing SCN5A, we identified novel common and rare coding variants associated with cardiac conduction.

Sodium channelopathy underlying familial sick sinus syndrome with early onset and predominantly male characteristics

BACKGROUND

Sick sinus syndrome (SSS) is a common arrhythmia often associated with aging or organic heart diseases but may also occur in a familial form with a variable mode of inheritance. Despite the identification of causative genes, including cardiac Na channel (SCN5A), the pathogenesis and molecular epidemiology of familial SSS remain undetermined primarily because of its rarity.

METHODS AND RESULTS

We genetically screened 48 members of 15 SSS families for mutations in several candidate genes and determined the functional properties of mutant Na channels using whole-cell patch clamping. We identified 6 SCN5A mutations including a compound heterozygous mutation. Heterologously expressed mutant Na channels showed loss-of-function properties of reduced or no Na current density in conjunction with gating modulations. Among 19 family members with SCN5A mutations, QT prolongation and Brugada syndrome were associated in 4 and 2 individuals, respectively. Age of onset in probands carrying SCN5A mutations was significantly less (mean±SE, 12.4±4.6 years; n=5) than in SCN5A-negative probands (47.0±4.6 years; n=10; P<0.001) or nonfamilial SSS (74.3±0.4 years; n=538; P<0.001). Meta-analysis of SSS probands carrying SCN5A mutations (n=29) indicated profound male predominance (79.3%) resembling Brugada syndrome but with a considerably earlier age of onset (20.9±3.4 years).

CONCLUSIONS

The notable pathophysiological overlap between familial SSS and Na channelopathy indicates that familial SSS with SCN5A mutations may represent a subset of cardiac Na channelopathy with strong male predominance and early clinical manifestations.

Atrial fibrillation-linked GJA5/connexin40 mutants impaired gap junctions via different mechanisms

The gap junctions (GJs) formed by Cx40 and Cx43 provide a low resistance passage allowing for rapid propagation of action potentials. Sporadic somatic mutations in GJA5 (encoding Cx40) have been identified in lone atrial fibrillation (AF) patients. More recently germline autosomal dominantly inherited mutations in GJA5 have been found in early onset lone AF patients in several families over generations. Characterizations of these AF-linked Cx40 mutants in model cells and in patient tissues revealed that some of the mutants reduced GJ channel function due to an impaired trafficking or channel formation. While others showed a gain-of-function in hemichannels. These functional alterations in GJs or hemichannel may play an important role in the pathogenesis of AF in the mutant carriers.

Biophysics, pathophysiology, and pharmacology of ion channel gating pores

Voltage sensor domains (VSDs) are a feature of voltage gated ion channels (VGICs) and voltage sensitive proteins. They are composed of four transmembrane (TM) segments (S1–S4). Currents leaking through VSDs are called omega or gating pore currents. Gating pores are caused by mutations of the highly conserved positively charged amino acids in the S4 segment that disrupt interactions between the S4 segment and the gating charge transfer center (GCTC). The GCTC separates the intracellular and extracellular water crevices. The disruption of S4–GCTC interactions allows these crevices to communicate and create a fast activating and non-inactivating alternative cation-selective permeation pathway of low conductance, or a gating pore. Gating pore currents have recently been shown to cause periodic paralysis phenotypes. There is also increasing evidence that gating pores are linked to several other familial diseases. For example, gating pores in Na_v1.5 and K_v7.2 channels may underlie mixed arrhythmias associated with dilated cardiomyopathy (DCM) phenotypes and peripheral nerve hyperexcitability (PNH), respectively. There is little evidence for the existence of gating pore blockers. Moreover, it is known that a number of toxins bind to the VSD of a specific domain of Na⁺ channels. These toxins may thus modulate gating pore currents. This focus on the VSD motif opens up a new area of research centered on developing molecules to treat a number of cell excitability disorders such as epilepsy, cardiac arrhythmias, and pain. The purpose of the present review is to summarize existing knowledge of the pathophysiology, biophysics, and pharmacology of gating pore currents and to serve as a guide for future studies aimed at improving our understanding of gating pores and their pathophysiological roles.

The SCN5A mutation A1180V is associated with electrocardiographic features of LQT3

Mutations of the SCN5A gene are associated with several arrhythmic syndromes including the Brugada syndrome, conduction disease, long QT syndrome type 3 (LQT3), atrial fibrillation, and dilated cardiomyopathy. We report LQT3 associated with an A1180V cardiac sodium channel mutation, previously associated with cardiac conduction block, and dilated cardiomyopathy in three generations of a Chinese family. Clinical, electrocardiographic (ECG), and echocardiographic examination was followed by direct sequencing of SCN5A and HERG to screen genomic DNA from blood samples. The proband presented with multiple syncopes from the age of 7 years and was found to share a mutation with two other members of his family. Continuous ECG monitoring after presentation showed prolonged QTc and biphasic T waves, multiple episodes of ventricular tachycardia and torsades de pointes. The other two mutation carriers showed ECG features of LQT3 without clinical symptoms. Transthoracic echocardiography showed normal cardiac structure in all three mutation carriers. This study shows LQT3 features associated with an A1180V cardiac sodium channel mutation, expanding the spectrum of phenotypes resulting from this mutation in which biophysical study has shown a persistent late Na(+) current.

Nav 1.5 mutations linked to dilated cardiomyopathy phenotypes: Is the gating pore current the missing link?

Nav 1.5 dysfunctions are commonly linked to rhythms disturbances that include type 3 long QT syndrome (LQT3), Brugada syndrome (BrS), sick sinus syndrome (SSS) and conduction defects. Recently, this channel protein has been also linked to structural heart diseases such as dilated cardiomyopathy (DCM).

Identifying potential functional impact of mutations and polymorphisms: linking heart failure, increased risk of arrhythmias and sudden cardiac death

Researchers and clinicians have discovered several important concepts regarding the mechanisms responsible for increased risk of arrhythmias, heart failure, and sudden cardiac death. One major step in defining the molecular basis of normal and abnormal cardiac electrical behavior has been the identification of single mutations that greatly increase the risk for arrhythmias and sudden cardiac death by changing channel-gating characteristics. Indeed, mutations in several genes encoding ion channels, such as SCN5A, which encodes the major cardiac Na⁺ channel, have emerged as the basis for a variety of inherited cardiac arrhythmias such as long QT syndrome, Brugada syndrome, progressive cardiac conduction disorder, sinus node dysfunction, or sudden infant death syndrome. In addition, genes encoding ion channel accessory proteins, like anchoring or chaperone proteins, which modify the expression, the regulation of endocytosis, and the degradation of ion channel a-subunits have also been reported as susceptibility genes for arrhythmic syndromes. The regulation of ion channel protein expression also depends on a fine-tuned balance among different other mechanisms, such as gene transcription, RNA processing, post-transcriptional control of gene expression by miRNA, protein synthesis, assembly and post-translational modification and trafficking. The aim of this review is to inventory, through the description of few representative examples, the role of these different biogenic mechanisms in arrhythmogenesis, HF and SCD in order to help the researcher to identify all the processes that could lead to arrhythmias. Identification of novel targets for drug intervention should result from further understanding of these fundamental mechanisms.

Cardiac sodium channelopathy associated with SCN5A mutations: electrophysiological, molecular and genetic aspects

Over the last two decades, an increasing number of SCN5A mutations have been described in patients with long QT syndrome type 3 (LQT3), Brugada syndrome, (progressive) conduction disease, sick sinus syndrome, atrial standstill, atrial fibrillation, dilated cardiomyopathy, and sudden infant death syndrome (SIDS). Combined genetic, electrophysiological and molecular studies have provided insight into the dysfunction and dysregulation of the cardiac sodium channel in the setting of SCN5A mutations identified in patients with these inherited arrhythmia syndromes. However, risk stratification and patient management is hindered by the reduced penetrance and variable disease expressivity in sodium channelopathies. Furthermore, various SCN5A-related arrhythmia syndromes are known to display mixed phenotypes known as cardiac sodium channel overlap syndromes. Determinants of variable disease expressivity, including genetic background and environmental factors, are suspected but still largely unknown. Moreover, it has become increasingly clear that sodium channel function and regulation is more complicated than previously assumed, and the sodium channel may play additional, as of yet unrecognized, roles in cardiac structure and function. Development of cardiac structural abnormalities secondary to SCN5A mutations has been reported, but the clinical relevance and underlying mechanisms are unclear. Increased insight into these issues would enable a major next step in research related to cardiac sodium channel disease, ultimately enabling improved diagnosis, risk stratification and treatment strategies.

Determinants of myocardial conduction velocity: implications for arrhythmogenesis

Slowed myocardial conduction velocity (θ) is associated with an increased risk of re-entrant excitation, predisposing to cardiac arrhythmia. θ is determined by the ion channel and physical properties of cardiac myocytes and by their interconnections. Thus, θ is closely related to the maximum rate of action potential (AP) depolarization [(dV/dt)_max], as determined by the fast Na⁺ current (I_Na); the axial resistance (r_a) to local circuit current flow between cells; their membrane capacitances (c_m); and to the geometrical relationship between successive myocytes within cardiac tissue. These determinants are altered by a wide range of pathophysiological conditions. Firstly, I_Na is reduced by the impaired Na⁺ channel function that arises clinically during heart failure, ischemia, tachycardia, and following treatment with class I antiarrhythmic drugs. Such reductions also arise as a consequence of mutations in SCN5A such as those occurring in Lenègre disease, Brugada syndrome (BrS), sick sinus syndrome, and atrial fibrillation (AF). Secondly, r_a, may be increased due to gap junction decoupling following ischemia, ventricular hypertrophy, and heart failure, or as a result of mutations in CJA5 found in idiopathic AF and atrial standstill. Finally, either r_a or c_m could potentially be altered by fibrotic change through the resultant decoupling of myocyte–myocyte connections and coupling of myocytes with fibroblasts. Such changes are observed in myocardial infarction and cardiomyopathy or following mutations in MHC403 and SCN5A resulting in hypertrophic cardiomyopathy (HCM) or Lenègre disease, respectively. This review defines and quantifies the determinants of θ and summarizes experimental evidence that links changes in these determinants with reduced myocardial θ and arrhythmogenesis. It thereby identifies the diverse pathophysiological conditions in which abnormal θ may contribute to arrhythmia.

A transgenic zebrafish model of a human cardiac sodium channel mutation exhibits bradycardia, conduction-system abnormalities and early death

The recent exponential increase in human genetic studies due to the advances of next generation sequencing has generated unprecedented numbers of new gene variants. Determining which of these are causative of human disease is a major challenge. In-vitro studies and murine models have been used to study inherited cardiac arrhythmias but have several limitations. Zebrafish models provide an attractive alternative for modeling human heart disease due to similarities in cardiac electrophysiology and contraction, together with ease of genetic manipulation, external development and optical transparency. Although zebrafish cardiac mutants and morphants have been widely used to study loss and knockdown of zebrafish gene function, the phenotypic effects of human dominant-negative gene mutations expressed in transgenic zebrafish have not been evaluated. The aim of this study was to generate and characterize a transgenic zebrafish arrhythmia model harboring the pathogenic human cardiac sodium channel mutation SCN5A-D1275N, that has been robustly associated with a range of cardiac phenotypes, including conduction disease, sinus node dysfunction, atrial and ventricular arrhythmias, and dilated cardiomyopathy in humans and in mice. Stable transgenic fish with cardiac expression of human SCN5A were generated using Tol2-mediated transgenesis and cardiac phenotypes were analyzed using video microscopy and ECG. Here we show that transgenic zebrafish expressing the SCN5A-D1275N mutation, but not wild-type SCN5A, exhibit bradycardia, conduction-system abnormalities and premature death. We furthermore show that SCN5A-WT, and to a lesser degree SCN5A-D1275N, are able to compensate the loss of endogenous zebrafish cardiac sodium channels, indicating that the basic pathways, through which SCN5A acts, are conserved in teleosts. This proof-of-principle study suggests that zebrafish may be highly useful in vivo models to differentiate functional from benign human genetic variants in cardiac ion channel genes in a time- and cost-efficient manner. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Prevalence and spectrum of GJA5 mutations associated with lone atrial fibrillation

Atrial fibrillation (AF) is the most common form of cardiac arrhythmia observed in clinical practice and a major contributor to cardiovascular morbidity and mortality. Accumulating evidence indicates a substantial genetic basis for AF. However, AF is genetically heterogeneous and the hereditary components responsible for AF remain to be identified in the majority of patients. The cardiac gap junction protein α 5 (GJA5) is specifically expressed in atrial myocytes and is associated with the coordinated electrical activation of the atria, providing a rationale to screen GJA5 as a logical candidate gene for AF. A cohort of 310 unrelated patients with lone AF and their available relatives were included in this study. A group of 200 unrelated healthy individuals matched for age, gender and race were also included as controls. The entire coding region and splice sites of the GJA5 gene were initially sequenced in 310 unrelated AF patients. The relatives of mutation carriers and 200 controls were subsequently genotyped for the presence of identified mutations. As a result, 4 novel heterozygous GJA5 mutations, p.K107R, p.L223M, p.Q236H and p.I257L, were identified in 4 of 310 unrelated AF patients, respectively, with a prevalence of ~1.29%. Genetic analysis of the carriers' families showed that in each family the missense mutation was present in all the affected family members. Absent in the 400 reference alleles, these mutations altered the amino acids highly conserved among various species, with the exception of p.I257L. In conclusion, this study expands the spectrum of GJA5 mutations associated with AF and provides novel insights into the molecular basis of AF, suggesting potential implications for the improved, gene-specific rhythm control strategies.

Genetics can contribute to the prognosis of Brugada syndrome: a pilot model for risk stratification

Brugada syndrome is an inherited arrhythmogenic disorder leading to sudden death predominantly in the 3-4 decade. To date the only reliable treatment is the implantation of a cardioverter defibrillator; however, better criteria for risk stratification are needed, especially for asymptomatic subjects. Brugada syndrome genetic bases have been only partially understood, accounting for <30% of patients, and have been poorly correlated with prognosis, preventing inclusion of genetic data in current guidelines. We designed an observational study to identify genetic markers for risk stratification of Brugada patients by exploratory statistical analysis. The presence of genetic variants, identified by SCN5A gene analysis and genotyping of 73 candidate polymorphisms, was correlated with the occurrence of major arrhythmic events in a cohort of 92 Brugada patients by allelic association and survival analysis. In all, 18 mutations were identified in the SCN5A gene, including 5 novel, and statistical analysis indicated that mutation carriers had a significantly increased risk of major arrhythmic events (P=0.024). In addition, we established association of five polymorphisms with major arrhythmic events occurrence and consequently elaborated a pilot risk stratification algorithm by calculating a weighted genetic risk score, including the associated polymorphisms and the presence of SCN5A mutation as function of their odds ratio. This study correlates for the first time the presence of genetic variants with increased arrhythmic risk in Brugada patients, representing a first step towards the design of a new risk stratification model.

Rare variants in GJA5 are associated with early-onset lone atrial fibrillation

BACKGROUND

Genetic factors are believed to be important in early-onset lone atrial fibrillation (AF). The gene GJA5 encodes the gap-junction protein Cx40, which together with Cx43 is responsible for the electrical coupling of the atrial cardiomyocytes. The regulatory single nucleotide polymorphism rs10465885 in GJA5 was recently associated with early-onset lone AF (< 60 years) and was also found to be strongly associated with Cx40 messenger RNA levels. We hypothesized that this gene would have a strong effect in patients with a more selected phenotype, and that the findings regarding rs10465885 could be replicated in this group.

METHODS

The coding region and flanking intron sequences of GJA5 were resequenced in 342 patients with onset of lone AF before the age of 50 (mean age at onset 34 ± 9 years), and in 216 controls. The single nucleotide polymorphism rs10465885 was genotyped in 342 patients and 534 control subjects and odds ratios were calculated for different genetic models.

RESULTS

Genotyping of rs10465885 showed that the patients with early-onset lone AF were more likely to carry the A allele compared with controls (odds ratio = 1.30; P = 0.011). When resequencing GJA5, we identified the mutation A96S, previously associated with lone AF, which was not present in our control subjects or in any publicly available database or the National Heart, Lung, and Blood Institute Exome Variant Server (NHLBI EVS; 10,758 alleles).

CONCLUSIONS

We show a highly significant association between the A allele of rs10465885 and onset of lone AF before age 50. This opposes a previous study, wherein the G allele was found to be associated with AF, and makes it impossible to exclude that the associations are coincidental.

Autosomal recessive atrial dilated cardiomyopathy with standstill evolution associated with mutation of Natriuretic Peptide Precursor A

BACKGROUND

Atrial dilatation and atrial standstill are etiologically heterogeneous phenotypes with poorly defined nosology. In 1983, we described 8-years follow-up of atrial dilatation with standstill evolution in 8 patients from 3 families. We later identified 5 additional patients with identical phenotypes: 1 member of the largest original family and 4 unrelated to the 3 original families. All families are from the same geographic area in Northeast Italy.

METHODS AND RESULTS

We followed up the 13 patients for up to 37 years, extended the clinical investigation and monitoring to living relatives, and investigated the genetic basis of the disease. The disease was characterized by: (1) clinical onset in adulthood; (2) biatrial dilatation up to giant size; (3) early supraventricular arrhythmias with progressive loss of atrial electric activity to atrial standstill; (4) thromboembolic complications; and (5) stable, normal left ventricular function and New York Heart Association functional class during the long-term course of the disease. By linkage analysis, we mapped a locus at 1p36.22 containing the Natriuretic Peptide Precursor A gene. By sequencing Natriuretic Peptide Precursor A, we identified a homozygous missense mutation (p.Arg150Gln) in all living affected individuals of the 6 families. All patients showed low serum levels of atrial natriuretic peptide. Heterozygous mutation carriers were healthy and demonstrated normal levels of atrial natriuretic peptide.

CONCLUSIONS

Autosomal recessive atrial dilated cardiomyopathy is a rare disease associated with homozygous mutation of the Natriuretic Peptide Precursor A gene and characterized by extreme atrial dilatation with standstill evolution, thromboembolic risk, preserved left ventricular function, and severely decreased levels of atrial natriuretic peptide.

Cardiac channelopathies: genetic and molecular mechanisms

Channelopathies are diseases caused by dysfunctional ion channels, due to either genetic or acquired pathological factors. Inherited cardiac arrhythmic syndromes are among the most studied human disorders involving ion channels. Since seminal observations made in 1995, thousands of mutations have been found in many of the different genes that code for cardiac ion channel subunits and proteins that regulate the cardiac ion channels. The main phenotypes observed in patients carrying these mutations are congenital long QT syndrome (LQTS), Brugada syndrome (BrS), catecholaminergic polymorphic ventricular tachycardia (CPVT), short QT syndrome (SQTS) and variable types of conduction defects (CD). The goal of this review is to present an update of the main genetic and molecular mechanisms, as well as the associated phenotypes of cardiac channelopathies as of 2012.

Rare copy number variations in adults with tetralogy of Fallot implicate novel risk gene pathways

Structural genetic changes, especially copy number variants (CNVs), represent a major source of genetic variation contributing to human disease. Tetralogy of Fallot (TOF) is the most common form of cyanotic congenital heart disease, but to date little is known about the role of CNVs in the etiology of TOF. Using high-resolution genome-wide microarrays and stringent calling methods, we investigated rare CNVs in a prospectively recruited cohort of 433 unrelated adults with TOF and/or pulmonary atresia at a single centre. We excluded those with recognized syndromes, including 22q11.2 deletion syndrome. We identified candidate genes for TOF based on converging evidence between rare CNVs that overlapped the same gene in unrelated individuals and from pathway analyses comparing rare CNVs in TOF cases to those in epidemiologic controls. Even after excluding the 53 (10.7%) subjects with 22q11.2 deletions, we found that adults with TOF had a greater burden of large rare genic CNVs compared to controls (8.82% vs. 4.33%, p = 0.0117). Six loci showed evidence for recurrence in TOF or related congenital heart disease, including typical 1q21.1 duplications in four (1.18%) of 340 Caucasian probands. The rare CNVs implicated novel candidate genes of interest for TOF, including PLXNA2, a gene involved in semaphorin signaling. Independent pathway analyses highlighted developmental processes as potential contributors to the pathogenesis of TOF. These results indicate that individually rare CNVs are collectively significant contributors to the genetic burden of TOF. Further, the data provide new evidence for dosage sensitive genes in PLXNA2-semaphorin signaling and related developmental processes in human cardiovascular development, consistent with previous animal models.

Congenital heart disease affects nearly 1% of all live births. Tetralogy of Fallot (TOF) is the most common form of cyanotic congenital heart disease. This condition is associated with hemizygous deletions of chromosome 22q11.2 and chromosomal trisomies, but little else is known about the genetic heterogeneity of this complex disease. We used high-resolution microarrays and stringent methods to study structural (copy number) variants in a systematically phenotyped cohort of unrelated adults with TOF. We found that individually rare genic copy number variants (CNVs) were collectively significant contributors to the genetic burden in TOF. Among CNVs that implicated candidate genes of interest were loss CNVs overlapping the PLXNA2 gene that codes for plexin A2. This is the first study to show a role for this semaphorin receptor in human congenital heart disease, consistent with a Plxna2 mouse knockout phenotype. Pathway analyses comparing rare exonic loss CNVs in the TOF sample to controls implicated other novel gene sets suggest new pathogenetic mechanisms.

An introduction to murine models of atrial fibrillation

Understanding the mechanism of re-entrant arrhythmias in the past 30 years has allowed the development of almost curative therapies for many rhythm disturbances. The complex, polymorphic arrhythmias of atrial fibrillation (AF) and sudden death are, unfortunately, not yet well understood, and hence still in need of adequate therapy. AF contributes markedly to morbidity and mortality in aging Western populations. In the past decade, many genetically altered murine models have been described and characterized. Here, we review genetically altered murine models of AF; powerful tools that will enable a better understanding of the mechanisms of AF and the assessment of novel therapeutic interventions.

SCN5A mutations in Brugada syndrome are associated with increased cardiac dimensions and reduced contractility

Background

The cardiac sodium channel (Na_v1.5) controls cardiac excitability. Accordingly, SCN5A mutations that result in loss-of-function of Na_v1.5 are associated with various inherited arrhythmia syndromes that revolve around reduced cardiac excitability, most notably Brugada syndrome (BrS). Experimental studies have indicated that Na_v1.5 interacts with the cytoskeleton and may also be involved in maintaining structural integrity of the heart. We aimed to determine whether clinical evidence may be obtained that Na_v1.5 is involved in maintaining cardiac structural integrity.

Methods

Using cardiac magnetic resonance (CMR) imaging, we compared right ventricular (RV) and left ventricular (LV) dimensions and ejection fractions between 40 BrS patients with SCN5A mutations (SCN5a-mut-positive) and 98 BrS patients without SCN5A mutations (SCN5a-mut-negative). We also studied 18 age/sex-matched healthy volunteers.

Results

SCN5a-mut-positive patients had significantly larger end-diastolic and end-systolic RV and LV volumes, and lower LV ejection fractions, than SCN5a-mut-negative patients or volunteers.

Conclusions

Loss-of-function SCN5A mutations are associated with dilatation and impairment in contractile function of both ventricles that can be detected by CMR analysis.

Reduced connexin40 protein expression in the right atrial appendage of patients bearing the minor connexin40 allele (-44 G --> A)

AIMS

The occurrence of connexin40 (Cx40) minor polymorphism (-44 G → A) was increased in patients with idiopathic atrial fibrillation (AF), although its effect on atrial Cx40 protein expression is unknown. We aimed to evaluate whether alterations in Cx40 are directly linked to the development of AF, we studied the effect of this polymorphism on Cx40 expression and distribution in patients without any history of AF and in patients who developed post-operative AF.

METHODS AND RESULTS

Hundred and eight patients (mean age 67 ± 9 years), without a history of AF or conditions that predispose to AF, were included. During heart surgery, 10 cc blood was collected for DNA genotyping and the right atrial appendage was partly excised. Ten patients (9%) were homozygous for the minor allele (AA, Group 1), 30 (28%) were heterozygous (AG, Group 2), and 68 (63%) were non-carriers (GG, Group 3). Ten age- and sex-matched tissue samples per group were analysed for Cx40 expression by: (i) real-time quantitative polymerase chain reaction (Q-PCR), (ii) western blotting, and (iii) immunohistochemistry on cryosections. Real-time quantitative polymerase chain reaction showed no significant differences of Cx40 mRNA among the groups. Western blot analysis, however, revealed a reduction in Cx40 protein in Groups 1 (-36.4%) and 2 (-39.5%) as compared with Group 3. Immunohistochemistry confirmed this reduction but indicated an unaltered subcellular distribution of the remaining Cx40. Incidence of post-operative AF (28%) was age-dependent but unrelated to the presence of the polymorphism or fibrosis.

CONCLUSION

Presence of the Cx40 minor allele (-44 G → A) results in a uniform down-regulation of right atrial appendage Cx40 protein which was not significantly related to development of post-operative AF.

Co-clustering phenome-genome for phenotype classification and disease gene discovery

Understanding the categorization of human diseases is critical for reliably identifying disease causal genes. Recently, genome-wide studies of abnormal chromosomal locations related to diseases have mapped >2000 phenotype–gene relations, which provide valuable information for classifying diseases and identifying candidate genes as drug targets. In this article, a regularized non-negative matrix tri-factorization (R-NMTF) algorithm is introduced to co-cluster phenotypes and genes, and simultaneously detect associations between the detected phenotype clusters and gene clusters. The R-NMTF algorithm factorizes the phenotype–gene association matrix under the prior knowledge from phenotype similarity network and protein–protein interaction network, supervised by the label information from known disease classes and biological pathways. In the experiments on disease phenotype–gene associations in OMIM and KEGG disease pathways, R-NMTF significantly improved the classification of disease phenotypes and disease pathway genes compared with support vector machines and Label Propagation in cross-validation on the annotated phenotypes and genes. The newly predicted phenotypes in each disease class are highly consistent with human phenotype ontology annotations. The roles of the new member genes in the disease pathways are examined and validated in the protein–protein interaction subnetworks. Extensive literature review also confirmed many new members of the disease classes and pathways as well as the predicted associations between disease phenotype classes and pathways.

Mouse Models of SCN5A-Related Cardiac Arrhythmias

Mutations of SCN5A gene, which encodes the α-subunit of the voltage-gated Na⁺ channel Na_V1.5, underlie hereditary cardiac arrhythmic syndromes such as the type 3 long QT syndrome, cardiac conduction diseases, the Brugada syndrome, the sick sinus syndrome, a trial standstill, and numerous overlap syndromes. Patch-clamp studies in heterologous expression systems have provided important information to understand the genotype-phenotype relationships of these diseases. However, they could not clarify how SCN5A mutations can be responsible for such a large spectrum of diseases, for the late age of onset or the progressiveness of some of these diseases and for the overlapping syndromes. Genetically modified mice rapidly appeared as promising tools for understanding the pathophysiological mechanisms of cardiac SCN5A-related arrhythmic syndromes and several mouse models have been established. This review presents the results obtained on these models that, for most of them, recapitulate the clinical phenotypes of the patients. This includes two models knocked out for Nav1.5 β1 and β3 auxiliary subunits that are also discussed. Despite their own limitations that we point out, the mouse models still appear as powerful tools to elucidate the pathophysiological mechanisms of SCN5A-related diseases and offer the opportunity to investigate the secondary cellular consequences of SCN5A mutations such as the expression remodeling of other genes. This points out the potential role of these genes in the overall human phenotype. Finally, they constitute useful tools for addressing the role of genetic and environmental modifiers on cardiac electrical activity.

Controlling Parasympathetic Regulation of Heart Rate: A Gatekeeper Role for RGS Proteins in the Sinoatrial Node

Neurotransmitters released from sympathetic and parasympathetic nerve terminals in the sinoatrial node (SAN) exert their effects via G-protein-coupled receptors. Integration of these different G-protein signals within pacemaker cells of the SAN is critical for proper regulation of heart rate and function. For example, excessive parasympathetic signaling can be associated with sinus node dysfunction (SND) and supraventricular arrhythmias. Our previous work has shown that one member of the regulator of G-protein signaling (RGS) protein family, RGS4, is highly and selectively expressed in pacemaker cells of the SAN. Consistent with its role as an inhibitor of parasympathetic signaling, RGS4-knockout mice have reduced basal heart rates and enhanced negative chronotropic responses to parasympathetic agonists. Moreover, RGS4 appears to be an important part of SA nodal myocyte signaling pathways that mediate G-protein-coupled inwardly rectifying potassium channel (GIRK) channel activation/deactivation and desensitization. Since RGS4 acts immediately downstream of M2 muscarinic receptors, it is tempting to speculate that RGS4 functions as a master regulator of parasympathetic signaling upstream of GIRKs, HCNs, and L-type Ca²⁺ channels in the SAN. Thus, loss of RGS4 function may lead to increased susceptibility to conditions associated with increased parasympathetic signaling, including bradyarrhythmia, SND, and atrial fibrillation.

A proton leak current through the cardiac sodium channel is linked to mixed arrhythmia and the dilated cardiomyopathy phenotype

Cardiac Na⁺ channels encoded by the SCN5A gene are essential for initiating heart beats and maintaining a regular heart rhythm. Mutations in these channels have recently been associated with atrial fibrillation, ventricular arrhythmias, conduction disorders, and dilated cardiomyopathy (DCM).

We investigated a young male patient with a mixed phenotype composed of documented conduction disorder, atrial flutter, and ventricular tachycardia associated with DCM. Further family screening revealed DCM in the patient's mother and sister and in three of the mother's sisters. Because of the complex clinical phenotypes, we screened SCN5A and identified a novel mutation, R219H, which is located on a highly conserved region on the fourth helix of the voltage sensor domain of Na_v1.5. Three family members with DCM carried the R219H mutation.

The wild-type (WT) and mutant Na⁺ channels were expressed in a heterologous expression system, and intracellular pH (pHi) was measured using a pH-sensitive electrode. The biophysical characterization of the mutant channel revealed an unexpected selective proton leak with no effect on its biophysical properties. The H⁺ leak through the mutated Na_v1.5 channel was not related to the Na⁺ permeation pathway but occurred through an alternative pore, most probably a proton wire on the voltage sensor domain.

We propose that acidification of cardiac myocytes and/or downstream events may cause the DCM phenotype and other electrical problems in affected family members. The identification of this clinically significant H⁺ leak may lead to the development of more targeted treatments.

Lack of association between connexin40 polymorphisms and coronary artery disease

OBJECTIVE

Cx40 is a gap junction protein important for cell-cell communication in the endothelium. Polymorphisms in the promoter region of the human Cx40 gene, -44G>A and +71A>G, were shown to reduce Cx40 transcription by half. As mice with an endothelial-specific deletion of Cx40 are more susceptible to atherosclerosis, this study was designed to discover a correlation between these polymorphisms and atherosclerosis in European populations.

METHODS AND RESULTS

803 patients referred to the Geneva University Hospitals for elective coronary angiography were divided according to the number of significantly stenosed vessels (from 0 to 3) and were genotyped for the Cx40 polymorphisms. Genotype distribution in the control group was -44GG/+71AA=59.8%, -44AG/+71AG=35.1% and -44AA/+71GG=5.2%. Surprisingly, this distribution was similar in the CAD group, with -44GG/+71AA=58.5%, -44AG/+71AG=37.6% and -44AA/+71GG=3.8% (p=0.67). Moreover, no significant association between histological carotid plaque composition of culprit lesions and Cx40 polymorphisms could be detected in 583 Dutch patients of the Athero-Express study.

CONCLUSIONS

Despite a clear antiatherogenic role of Cx40 in mice, our study could not detect an association of Cx40 promoter polymorphisms and CAD in human. Moreover, a correlation with atherosclerotic plaque stability or hypertension could not be demonstrated either. Connexin polymorphisms affecting channel function may be of greater importance for cardiovascular disease than polymorphisms affecting the expression level of the protein.

Altered sinoatrial node function and intra-atrial conduction in murine gain-of-function Scn5a+/ΔKPQ hearts suggest an overlap syndrome

Mutations in SCN5A, the gene encoding the pore-forming subunit of cardiac Na⁺ channels, cause a spectrum of arrhythmic syndromes. Of these, sinoatrial node (SAN) dysfunction occurs in patients with both loss- and gain-of-function SCN5A mutations. We explored for corresponding alterations in SAN function and intracardiac conduction and clarified possible mechanisms underlying these in an established mouse long QT syndrome type 3 model carrying a mutation equivalent to human SCN5A-ΔKPQ. Electrophysiological characterizations of SAN function in living animals and in vitro sinoatrial preparations were compared with cellular SAN and two-dimensional tissue models exploring the consequences of Scn5a+/ΔKPQ mutations. Scn5a+/ΔKPQ mice showed prolonged electrocardiographic QT and corrected QT intervals confirming long QT phenotypes. They showed frequent episodes of sinus bradycardia, sinus pause/arrest, and significantly longer sinus node recovery times, suggesting compromised pacemaker activity compared with wild-type mice. Electrocardiographic waveforms suggested depressed intra-atrial, atrioventricular node, and intraventricular conduction in Scn5a+/ΔKPQ mice. Isolated Scn5a+/ΔKPQ sinoatrial preparations similarly showed lower mean intrinsic heart rates and overall slower conduction through the SAN to the surrounding atrium than did wild-type preparations. Computer simulations of both single SAN cells as well as two-dimensional SAN-atrial models could reproduce the experimental observations of impaired pacemaker and sinoatrial conduction in terms of changes produced by both augmented tail and reduced total Na⁺ currents, respectively. In conclusion, the gain-of-function long QT syndrome type 3 murine Scn5a+/ΔKPQ cardiac system, in overlap with corresponding features reported in loss-of-function Na⁺ channel mutations, shows compromised SAN pacemaker and conduction function explicable in modeling studies through a combination of augmented tail and reduced peak Na⁺ currents.

Connexins: key mediators of endocrine function

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing β-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

SCN5A mutations associate with arrhythmic dilated cardiomyopathy and commonly localize to the voltage-sensing mechanism

OBJECTIVES

The aim of this study was to discern the role of the cardiac voltage-gated sodium ion channel SCN5A in the etiology of dilated cardiomyopathy (DCM).

BACKGROUND

Dilated cardiomyopathy associates with mutations in the SCN5A gene, but the frequency, phenotype, and causative nature of these associations remain the focus of ongoing investigation.

METHODS

Since 1991, DCM probands and family members have been enrolled in the Familial Cardiomyopathy Registry and extensively evaluated by clinical phenotype. Genomic deoxyribonucleic acid samples from 338 individuals among 289 DCM families were obtained and screened for SCN5A mutations by denaturing high-performance liquid chromatography and sequence analysis.

RESULTS

We identified 5 missense SCN5A mutations among our DCM families, including novel mutations E446K, F1520L, and V1279I, as well as previously reported mutations D1275N and R222Q. Of 15 SCN5A mutation carriers in our study, 14 (93%) manifested arrhythmia: supraventricular arrhythmia (13 of 15), including sick sinus syndrome (5 of 15) and atrial fibrillation (9 of 15), ventricular tachycardia (5 of 15), and conduction disease (9 of 15).

CONCLUSIONS

Mutations in SCN5A were detected in 1.7% of DCM families. Two-thirds (6 of 9) of all reported DCM mutations in SCN5A localize to the highly conserved homologous S3 and S4 transmembrane segments, suggesting a shared mechanism of disruption of the voltage-sensing mechanism of this channel leading to DCM. Not surprisingly, SCN5A mutation carriers show a strong arrhythmic pattern that has clinical and diagnostic implications.

Striking In vivo phenotype of a disease-associated human SCN5A mutation producing minimal changes in vitro

BACKGROUND

The D1275N SCN5A mutation has been associated with a range of unusual phenotypes, including conduction disease and dilated cardiomyopathy, as well as atrial and ventricular tachyarrhythmias. However, when D1275N is studied in heterologous expression systems, most studies show near-normal sodium channel function. Thus, the relationship of the variant to the clinical phenotypes remains uncertain.

METHODS AND RESULTS

We identified D1275N in a patient with atrial flutter, atrial standstill, conduction disease, and sinus node dysfunction. There was no major difference in biophysical properties between wild-type and D1275N channels expressed in Chinese hamster ovary cells or tsA201 cells in the absence or presence of β1 subunits. To determine D1275N function in vivo, the Scn5a locus was modified to knock out the mouse gene, and the full-length wild-type (H) or D1275N (DN) human SCN5A cDNAs were then inserted at the modified locus by recombinase mediated cassette exchange. Mice carrying the DN allele displayed slow conduction, heart block, atrial fibrillation, ventricular tachycardia, and a dilated cardiomyopathy phenotype, with no significant fibrosis or myocyte disarray on histological examination. The DN allele conferred gene-dose-dependent increases in SCN5A mRNA abundance but reduced sodium channel protein abundance and peak sodium current amplitudes (H/H, 41.0±2.9 pA/pF at -30 mV; DN/H, 19.2±3.1 pA/pF, P<0.001 vs. H/H; DN/DN, 9.3±1.1 pA/pF, P<0.001 versus H/H).

CONCLUSIONS

Although D1275N produces near-normal currents in multiple heterologous expression experiments, our data establish this variant as a pathological mutation that generates conduction slowing, arrhythmias, and a dilated cardiomyopathy phenotype by reducing cardiac sodium current.

SCN5A rare variants in familial dilated cardiomyopathy decrease peak sodium current depending on the common polymorphism H558R and common splice variant Q1077del

Obtaining functional data with newly identified rare variants increases certainty that the variant identified is relevant for dilated cardiomyopathy (DCM) causation. Two novel SCN5A rare variants, R222Q and I1835T, segregated with DCM in two families with affected individuals homozygous or heterozygous for the common SCN5A polymorphism H558R. cDNAs with each rare variant were constructed in the common Q1077del or Q1077 splice variant backgrounds with and without the H558R polymorphism and expressed in HEK293 cells. Sodium current (I(Na) ) was studied for each using whole-cell voltage clamp. In the Q1077del background I(Na) densities of R222Q and I1835T were not different from wild type, but the combined variants of R222Q/H558R, I1835T/H558R caused approximately 35% and approximately 30% reduction, respectively, and each showed slower recovery from inactivation. In the Q1077del background R222Q and R222Q/H558R also exhibited a significant negative shift in both activation and inactivation while I1835T/H558R showed a significant negative shift in inactivation that tended to decrease window current. In contrast, expression in the Q1077 background showed no changes in peak I(Na) densities, decay, or recovery from inactivation for R222Q/H558R and I1835T/H558R. We conclude that the biophysical findings, dependent upon common SCN5A variants, provide further evidence that these novel SCN5A rare variants are relevant for DCM.

A common connexin-40 gene promoter variant affects connexin-40 expression in human atria and is associated with atrial fibrillation

BACKGROUND

A common single-nucleotide polymorphism (SNP) in the promoter of the Connexin-40 (Cx40) gene GJA5 was suggested to affect Cx40 promoter activity and the risk of atrial fibrillation (AF), but the role of other common Cx40 polymorphisms is unknown.

METHODS AND RESULTS

Eight SNPs within the Cx40 gene region were tested for association with Cx40 levels measured in atrial tissue from 61 individuals. The previously described Cx40 promoter SNP (rs35594137, -44G→A) was not associated with Cx40 mRNA levels. However, a common SNP (rs10465885) located in the TATA box of an alternative Cx40 promoter was strongly associated with Cx40 mRNA expression (P<0.0001) and displayed strong and consistent allelic expression imbalance in human atrial tissue. A promoter-luciferase assay in cultured murine cardiomyocytes demonstrated reduced activity of the promoter containing the minor allele of this SNP (P<0.0001). Both rs35594137 and rs10465885 were tested for association with early-onset lone AF (≤60 years of age) in 384 cases and 3010 population control subjects. rs10465885 was associated with the AF phenotype (odds ratio, 1.18; P=0.046). This result was confirmed in a meta-analysis including 2 additional early-onset lone AF case-control cohorts (odds ratio, 1.16, P=0.022). rs35594137 was not associated with the lone AF phenotype in any of the cohorts studied or in a combined analysis.

CONCLUSIONS

A previously described Cx40 promoter SNP was not found to influence Cx40 expression or risk of AF. We describe an alternate promoter polymorphism that directly affects levels of Cx40 mRNA in vivo and is associated with early-onset lone AF.

Progressive Conduction Diseases

Progressive cardiac conduction disease (PCCD), a source of considerable morbidity, comprises a heterogeneous group of conditions resulting from genetic predisposition, environmental modifiers, and other physiologic determinants, including aging. The genetic factors include numerous mutations and variants within the cardiac sodium channel gene, SCN5A. The electrocardiographic phenotype has variable penetrance and is associated with appearances ranging from an isolated conduction disorder to an association with tachyarrhythmias and clinically significant cardiomyopathy. A heterozygotic Scn5a mouse model provides evidence that PCCD may lead to cardiac remodeling consistent with clinical observations in addition to slowing of intracardiac conduction. PCCD has also been associated with the altered expression of genes encoding other proteins involved in impulse propagation, including those responsible for Ca^2+- activated ion channels and cytoskeletal components, both in the presence or absence of structural abnormalities.

Monogenic atrial fibrillation as pathophysiological paradigms

Atrial fibrillation (AF) is the most common cardiac rhythm abnormality and represents a major burden, both to patients and to health-care systems. In recent years, increasing evidence from population-based studies has demonstrated that AF is a heritable condition. Although familial forms of AF have been recognized for many years, they represent a rare subtype of the arrhythmia. However, despite their limited prevalence, the identification of mutations in monogenic AF kindreds has provided valuable insights into the molecular pathways underlying the arrhythmia and a framework for investigating AF encountered in the general population. In contrast to these rare families, the typical forms of AF occurring in the community are likely to be multigenic and have significant environmental influences. Recently, genome-wide association studies have uncovered common sequence variants that confer increased susceptibility to the arrhythmia. In the future, the elucidation of the genetic substrate underlying both familial and more typical forms of AF will hopefully lead to the development of novel diagnostic tools as well as more targeted rhythm control strategies. In this article, we will focus on monogenic forms of AF and also provide an overview of case-control association studies for AF.